The present invention relates to a spray powder for the manufacture of layers having a high resistance to wear and burn traces to be used as coatings on the bearing surfaces of engine parts subject to friction, and more particularly, to a spray powder for forming a coating for the bearing surfaces of piston rings used in medium and large piston engines for operation with heavy lubricants.
Piston rings of internal-combustion engines are subjected to high wear and heat stresses during operation. Therefore, in the past, the bearing surfaces of piston rings have been coated with protective coatings according to known prior art processes. Such coatings have mainly consisted of electrochemically applied hard chromium layers or molybdenum layers which are applied by thermal spray processes. However, with medium and large piston engines which operate with heavy lubricants, it happens again and again that the engines fail because of excess piston ring wear. Further, the manufacturers of such engines have increased average pressures and numbers of revolutions, and this has given rise to additional difficulties because of the formation of burn traces in the starting phase or after a short period of operation, respectively.
Attempts have therefore been made to electrochemically apply thicker chromium layers to the bearing surfaces of the piston rings. The high inherent stresses of such coatings, however, produce macrocracks and these again, under dynamic stresses, result in permanent breaks and thus loss of the rings.
Experiments have been made with thermally applied sprayed layers. However, piston rings usually employed in an engine and sprayed with molybdenum completely failed due to their low wear resistance. More favorable results have been obtained with known layers based on a mixture of molybdenum and chromium carbide, which mixture is applied by a plasma spraying process. Although wear resistance was satisfactory here, the burn trace behavior was not.